An image-forming device using an electrophotographic system such as a copying machine or a laser printer transfers a toner image formed on a photoreceptive drum to a sheet of recording medium, passes the sheet with the toner image thereon through a fuser and feeds out a result. In such an image-forming device, a sheet supplied from the feeding portion provided with a feed tray is transported to an image transfer section by a transport roller device provided in the sheet transport path, and positioning of the leading edge of the sheet is carried out by a sheet registration system disposed immediately before the image transfer section. A sheet is fed in synchronism with the formation of the toner image on the photoreceptive drum and the toner image is transferred.
In an image-forming device of a general copying machine, a sheet supplied from a feed tray is transported being held by a transport roller device disposed in the sheet transport path. The transport roller device in the sheet transport path may, however, have inaccuracies in the positioning of their axes, or the rollers of rubber or the like may not be precisely circular, or the pressure of the transport roller device may not be even in the lateral direction of the transport path. Further, in transporting a sheet held by the rollers, skewing of the sheet may occur.
To prevent this, a sheet registration system is disposed immediately before the image transfer section, and in the image transfer section, the sheet is brought in registration with a reference line. The sheet registration system comprises a combination of registration roller device and gate members disposed immediately before the photosensitive drum and a preregistration roller device disposed upstream of the registration roller device with a specified spacing. The leading edge of a sheet transported in the sheet transport path is stopped by the gate members and the sheet is curved by additional feeding of the preregistration roller device. The curve of the sheet presses the leading edge of the sheet against the gate members uniformly and thus registration of the leading edge of the sheet is carried out so as to prevent skewing of the sheet transported by the registration system toward the image transfer section.
To register a sheet in the image transfer section, Japanese unexamined patent publication Hei 3-94275 (1991) describes the provision of a sheet registration system. A sheet registration system shown in the conventional examples has, as shown in FIG. 19, a sheet registration system 10 comprising a registration roller device 11 and a preregistration roller device 12, and gate members 3 selectively appearing in the sheet transport path are disposed immediately before the registration roller device 11. The registration roller device 11 comprises a drive axis 11a having a plurality of rollers 11b installed thereon, and a driven gear 18 and a lateral movement mechanism 15 both disposed at an end of the drive axis 11a.
The lateral movement mechanism 15 comprising a driven gear member 16 disposed around an end of the drive axis and a drive gear 17 provided on the body of the unit allows the drive axis to move reciprocally in the direction of the arrow y. The driven gear 16 allows the drive axis to rotate freely but constrains it to move axially. A sheet detector Sr is disposed at the other end of the drive axis, and it controls the movement in the lateral direction of the drive axis, in registering a sheet, by moving registration roller device 11 laterally as the leading edge of the sheet is held. The preregistration roller device 12 disposed upstream of the registration roller device 11 comprises a drive axis 12a having a plurality of rollers 12b installed thereon.
In the sheet registration system 10 shown in FIG. 19, if a sheet is transported skewed or off the reference line K, the sheet detector Sp detects it and makes the registration roller device 11 move in the direction lateral to the sheet transport path, corresponding to the detection. For example, if a sheet is transported off the reference line K as shown in the figure, the leading edge Pa of the sheet is stopped by the gate members 3, and the sheet is curved by the additional feeding of the preregistration roller device 12. Registration of the sheet is carried out by releasing the gate members 3, causing the leading edge of the sheet to be held by the registration roller device 11, stopping the leading edge of the sheet at the position shown as Pb in the figure and moving the registration roller device 11 in the direction lateral to the sheet transport path by means of the lateral movement mechanism 15 to the position where the side of the sheet is detected by the sheet detector Sp.
After being thus registered, the sheet is fed out from the registration system with a feed-out time matched with the formation of a toner image on the photoreceptive drum. Thus, positioning of the sheet fed from the sheet registration system is carried out by making its side brought in registration with the reference line K, which prevent dislocation of the toner image transferred to the sheet.
As described so far, in the registration system comprising a registration roller device movable along its axis and a preregistration roller device disposed at a specified position, a sheet moved in the direction lateral to the transport path by the registration roller device is subject to a shear stress between the registration roller device and the preregistration roller device which holds the sheet at the specified position. The further the sheet is moved in the lateral direction of the path by the registration roller device, the more shear stress is applied, and this may lead to wrinkling of sheet.
After being released from the preregistration roller device, the sheet is fed only by the registration roller device, and this may cause incomplete location of a transferred toner image, because the lateral reaction force of the sheet varies and the sheet is moved in the direction lateral to the transport path.
To prevent this, for example, the provision of a means which allows the preregistration roller device 12 to move in the direction lateral to the sheet transport path as shown in FIG. 20 has been proposed. When the sheet is moved in the direction lateral to the path, the springs 13 and 13a disposed at both ends of the preregistration roller device expand or contract with the shear reaction force of the sheet, thus eliminating the shear reaction force in the sheet. Whenever registration of a sheet is carried out, the springs 13 and 13a expand and contract, and the repetition deteriorates the spring, so that the preregistration roller device fails to return to its correct position after repeated use.
The sheet registration system shown in FIG. 19 is disposed corresponding to the image transfer section. In a color copying machine, in disposing a sheet registration system in the place supplying a sheet to the transfer drum, the sheet is subject to a shear stress. For example, Japanese unexamined patent publication Hei 2-238480 (1990) describes a color copying machine in which a photoreceptive drum is disposed facing a transfer drum holding a sheet, the photoreceptive drum has a plurality of color developers and color images formed on the photoreceptive drum are transferred in sequence and superimposed on the sheet held by the transfer drum.
To supply a sheet to the transfer drum, a sheet registration system as shown in FIG. 21 is provided. The sheet registration system comprises a registration roller device 11 and a preregistration roller device 12 similar to those shown in FIG. 19, and it makes the sheet P curve and registers the leading edge of the sheet and then supplies it to the transfer drum. In the example shown in FIG. 21, however, in registering and feeding out a sheet, a shear stress in the direction shown by an arrow F may be applied to the sheet P.
Being fed out by the two rollers devices, the sheet is pressed against the transfer drum 5 off the reference line K, which may distort the film provided as a sheet retaining member on the transfer drum. When the trailing edge of the sheet is released from the preregistration roller device, the film distorting force of the sheet is released, and the reaction force of the film dislocates the sheet in the direction of the reference line K. In a color copying machine, if transfer of the first color toner image is started while the sheet is being fed to the transfer drum, the sheet may move in the lateral direction while transferring the toner image of the first color, which leads to deterioration of the image quality.